Optical signal sources have a significant phase noise. When two optical sources are combined, an additional signal is produced in a noise band around a center frequency, (w1-w2). If the frequency range of this unwanted signal band overlaps with wanted signals, the signal to noise ratio of the wanted signal may be severely impacted. This is called optical beat interference (OBI) and is a practical problem, particularly in cable television (CATV) return systems, where multiple optical signals are combined on a single detector.